If you go and file it in person you can do it 15 days before or 7 days after the due date. Usually the first report has to be done at the immigration office.
If you are still before the due date and it's not your first 90 day report you can also do it online:
The plans of Pacific Cross are decent. If possible, I would recommend to get at least the Maxima Plus plan. Better to go for Ultima or Ultima Plus. The outpatient coverage of PC is pretty good and affordable compared to other insurers. They also guarantee lifelong renewals.
They have a premium system where they give no-claim discounts. However, in case of a claim that discount will be cancelled and depending on the claim amount and total premium payment they might load the premium with up to 25%. This is just something to be aware of.
In the event of a claim, they are not the easiest to deal with, but overall they will pay if the claim is justified - sometimes after some back and forth.
Depending on your age and budget, I might be able to offer you better solutions.
I had no issues getting the document from a Bangkok Bank branch in Hua Hin even though my main branch, the one where I initially opened the account, was in Bangkok.
If you want, you can send me your account number(s). I‘d assume that if I start a transfer to it and it shows your name, the account would still be active.
Merchant ID sounds more like business internet banking.